This invention relates generally to optical components and more particularly to optical components for correction of perspective distortion.
Over the course of history many different measurement standards have been developed. In one kingdom, the use of the king""s foot was used as a standard and in another kingdom the use of another kings foot was used as a standard. A problem arose when someone from one kingdom communicated a distance, in feet, to someone in another kingdom. This was solved with the use of international standard units for measurement.
In some biometric measurements devices a measured biometric template is only comparable to a biometric template measured on the same or a similar system. One reason that measurements are not standardized is that the biometric template is a perturbed image and not a xe2x80x9ctruexe2x80x9d representation of its biometric surface. This situation is particularly evident in optical finger print readers.
To overcome this problem, some systems employ software algorithms for correcting images of biometric surfaces. Unfortunately, software algorithms either rely on culling information which reduces the overall information content of an image or on extrapolation which adds unreliable information to an image. It would be very beneficial to provide a system that overcomes these disadvantages.
In an optical fingerprint input transducer or sensor, the finger under investigation is usually pressed against a flat surface, a platen, such as a side of a glass plate, and the ridge and valley pattern of the finger tip is sensed by a sensing means such as an interrogating light beam. Various optical devices are known which employ prisms upon which a finger whose print is to be identified is placed. The prism has a first surface, the platen, upon which a finger is placed, a second surface disposed at an acute angle to the first surface through which the fingerprint is viewed and a third illumination surface through which light is directed into the prism. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example, in U.S. Pat. Nos. 5,109,427 and 5,233,404. Fingerprint identification devices of this type are generally used to control the building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
One reason for the use of software correction of fingerprint data from optical finger print sensors is the use of angled surfaces. The use of the angled surfaces induces a perspective effect to measured biometric data. This perspective effect is very much the same as when a picture is angled away from a viewer. Pictorial objects of the same size on the picture appear to be of different sizes.
It has now been found that it is possible to correct for perspective distortion in optical fingerprint readers without a substantial reduction in imaged information. By providing a more accurate representation of a biometric surface using corrective optics, it is now possible to provide biometric templates having higher resolution relative to an image sensor used.
Thus, in an attempt to overcome these and other limitations of prior art devices, it is an object of this invention to provide an improved optical fingerprint-imager. It is also an object of the invention to provide corrective optical component for use in an optical contact imager.
Thus, in accordance with the present invention, an optical contact imaging device is provided comprising: an input port; a platen having a surface for receipt of a finger surface; an output port; a perspective correction optical component; a detector; and, an interrogating light source for launching light into the input port such that the light is incident on and reflected by the platen surface, emerges from the output port and is incident on the detector to provide a detected signal, wherein the light is perturbed by the perspective correction optical component such that perspective distortion is reduced.
In accordance with another aspect of the invention, a perspective correction optical component is provided comprising: a platen; an input port for receipt of light such that the light reflects off the platen; and, an output port for light reflected off the platen to emerge, wherein the output port comprises a complex surface and the complex surface is for correcting perspective distortion.
In accordance with the invention there is further provided, a perspective correction optical component comprising: a platen for receipt of a fingertip, wherein the platen comprises a recessed complex surface and the recessed complex surface is for reducing perspective distortion; an input port for receipt of light such that the light reflects off the platen; and, an output port for light reflected off the platen to emerge.
In accordance with the invention there is further provided a method of determining a perspective distortion correction for an optical fingerprint-imager having a platen and an image sensor such that light incident on and reflected by the platen is incident on the detector, the method comprising the steps of: (i) taking a first scan line on the platen; (ii) correlating the first scan line on the platen with a first image line on the detector; (iii) determining an optically corrective equation, describing an optical contour of an optical component, which correlates the first scan line on the platen with a second image line on the detector, wherein the first a scan line on the platen is proportional to the second image line on the detector.
In accordance with the invention there is still further provided a method of determining a perspective distortion correction for an optical fingerprint-imager having a platen and an image sensor such that light incident on and reflected by the platen is incident on the detector, the method comprising the steps of: (i) correlating a first reading coordinate on the platen with a first image coordinate on the image sensor; (ii) correlating a second reading coordinate on the platen with a second image coordinate on the image sensor; (iii) correlating a third reading coordinate on the platen with a third image coordinate on the image sensor; (iv) determining a two dimensional relationship between the first, second and third reading coordinates on the platen; (v) determining a two dimensional relationship between the first, second and third image coordinates on the image sensor; and, (v) determining contours of an optical surface for perturbing the light such that the first, second and third reading coordinates on the platen are substantially proportional to the first, second and third image coordinates on the image sensor.